Answer step defaults to low reasoning effort (issue #134)

Product chose A after the A/B: low cut answer-step reasoning tokens ~45%
and answer time 34s -> 22s with no quality loss. Unset now means "low";
CONSULT_ANSWER_REASONING_EFFORT=off restores the pre-change request body,
and the old "unset keeps the body" guarantee moves to "off".

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

frontend/tests/answer-effort-switch-20261013.test.ts

@@ -51,9 +51,12 @@ test("an illegal effort warns once and stays unset", () => {
   }
 });
 
-test("unset effort keeps the pre-change request bodies", async () => {
+// 原值: 不设环境变量时请求体与改动前逐字节相同
+// 新值: 设成 "off" 时请求体与改动前逐字节相同；不设时默认 "low"（见下一条）
+// 原因: issue #134 产品选 A，答题步默认低思考量；"off" 是一行回退
+test("off keeps the pre-change request bodies", async () => {
   const previous = process.env.CONSULT_ANSWER_REASONING_EFFORT;
-  delete process.env.CONSULT_ANSWER_REASONING_EFFORT;
+  process.env.CONSULT_ANSWER_REASONING_EFFORT = "off";
   try {
     const natal = await captureStreamBodies({
       settings: consultationGenerationSettings(MODEL),
@@ -76,6 +79,43 @@ test("unset effort keeps the pre-change request bodies", async () => {
   }
 });
 
+test("unset or blank effort defaults to low", () => {
+  const previous = process.env.CONSULT_ANSWER_REASONING_EFFORT;
+  try {
+    delete process.env.CONSULT_ANSWER_REASONING_EFFORT;
+    assert.equal(consultAnswerReasoningEffort(), "low");
+    process.env.CONSULT_ANSWER_REASONING_EFFORT = "  ";
+    assert.equal(consultAnswerReasoningEffort(), "low");
+    process.env.CONSULT_ANSWER_REASONING_EFFORT = "off";
+    assert.equal(consultAnswerReasoningEffort(), undefined);
+  } finally {
+    if (previous === undefined) delete process.env.CONSULT_ANSWER_REASONING_EFFORT;
+    else process.env.CONSULT_ANSWER_REASONING_EFFORT = previous;
+  }
+});
+
+test("unset effort puts low on the answer step only", async () => {
+  const previous = process.env.CONSULT_ANSWER_REASONING_EFFORT;
+  delete process.env.CONSULT_ANSWER_REASONING_EFFORT;
+  try {
+    const natal = await captureStreamBodies({
+      settings: consultationGenerationSettings(MODEL),
+      prepareStep: (input) => consultationNatalPrepareStep({
+        stepNumber: input.stepNumber,
+        answerReady: input.stepNumber >= 1,
+        model: MODEL,
+      }),
+      steps: 2,
+    }) as Record<string, unknown>[];
+    assert.deepEqual(natal[0], fixture.natal[0]);
+    assert.equal(natal[1]?.reasoning_effort, "low");
+    assert.deepEqual(withoutEffort(natal[1]), fixture.natal[1]);
+  } finally {
+    if (previous === undefined) delete process.env.CONSULT_ANSWER_REASONING_EFFORT;
+    else process.env.CONSULT_ANSWER_REASONING_EFFORT = previous;
+  }
+});
+
 test("low effort is only on the answer step", async () => {
   const previous = process.env.CONSULT_ANSWER_REASONING_EFFORT;
   process.env.CONSULT_ANSWER_REASONING_EFFORT = "low";